leds: qpnp-flash-v2: Fix low current configuration for PM8150L

PM8150L has a HW issue that prevents the flash LED module from detecting
a valid open fault if LED is disconnected and target current is set to
0. As per the hardware recommendation, fix this by changing IRES to
5 mA and calculating the current code for that IRES when the target
current set is equal to or below 20 mA.

CRs-Fixed: 2319765
Change-Id: I9b2fc0e790d89654ea1bed31ab6edbe05491e8fa
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>